In the related art, a sheet heating device which supplies a large amount of electric power to a heater to quickly raise a temperature thereof to a predetermined temperature during start-up, image fixing, or image decoloring is known. In the above conventional sheet heating device, because a large amount of electric power is supplied, the variation in the resulting heater temperature of the heater tends to be large.
For example, when starting up the sheet heating device after the sheet heating device has been turned off or has gone into a sleep mode, a large amount of electric power is supplied to the heat source to quickly raise the heater temperature to bring the sheet heating device to a warmed-up state. The large amount of electric power supplied may, however, cause overshooting of the heater temperature, because the heater temperature at the time of the start-up may be higher than its normal powered-off temperature. This overshooting is undesirable because it results in unnecessary power consumption.
Overshooting may also occur when raising the heater temperature of the sheet heating device to perform image fixing or image decoloring. The overshooting that results during this process is undesirable because when sheets are subjected to temperatures that are much higher than a target fixing or decoloring temperature, melted toner may remain on the sheets after they have been processed to cause the sheets to adhere to each other after they are discharged. In some cases, the overshooting may even cause image fixation failure.